Power control interface between a wind farm and a power transmission system

ABSTRACT

A power control interface between an unstable power source such as a wind farm and a power transmission line employs an electrical energy storage, control system, and electronic compensation module which act together like an “electronic shock absorber” for storing excess power during periods of increased power generation and releasing stored energy during periods of decreased power generation due to wind fluctuations. The control system is provided with a “look ahead” capability for predicting power output (wind speed conditions) and maintaining energy storage or release over a “narrow-band” range despite short duration fluctuations. The control system uses data derived from monitoring the wind farm power output and the power transmission line, and employs system-modeling algorithms to predict narrow-band wind speed conditions. The power control interface can also use its energy storage capacity to provide voltage support at the point of injection into the power transmission system, as well as fault clearance capability for “riding out” transient fault conditions occurring on the power transmission line.

This U.S. patent application claims the priority benefit of U.S.Provisional Application No. 60/435,643 of the same inventor, filed onDec. 20, 2002.

TECHNICAL FIELD

This invention generally relates to a power control interface between apower source and a power transmission system, and more particularly, toan interface between the power output of an unstable power source suchas a wind farm and a power transmission grid.

BACKGROUND OF INVENTION

Power generation using wind-driven windmills (turbines) is usable inareas that have good wind resources and that can benefit from theaddition of wind-generated power into a local power transmission system(often referred to as “the grid”). However, wind turbines are relativelyunstable power sources that fluctuate with wind conditions and must beproperly interfaced to avoid carrying over instabilities into the grid.A wind farm connected to either a weakly supported transmission line orto a relatively small transmission system (such as for an isolatedregion or island) can inject instabilities in both voltage and frequencyof the backbone transmission system because of the gusty and turbulentnature of the wind source. Even on a more robust, interconnectedtransmission system, such instabilities can create disturbances thatpropagate through the system.

As wind changes velocity over the area of the wind farm and interactswith individual windmills over varying time periods, and/or turbulentwind flow is created by passing weather systems, the energy output ofthe wind farm can change very rapidly—over a period of one second orless. This change in energy output of the wind farm is reflected bychanges in both frequency and voltage in the transmission grid to whichthe wind farm is connected. In extreme cases, these fluctuations maybecome large enough that it is necessary to disconnect the wind farmfrom the transmission system and simply waste the wind energy. Suchconditions have a strong economic impact on a wind farm, which recoverscosts only when electricity is being generated. Under less extremeconditions, the shifting winds create energy surges that are reflectedin lower-level voltage and frequency disturbances on the transmissionsystem—over a period of 1-2 minutes.

To maintain transmission system stability under these circumstances,compensation is conventionally provided by load-following of theunstable power source with larger capacities of more stable generationunits, such as fuel-fired or “thermal” generators. However, suchload-following can subject these other units to excessive internalmechanical and thermal fatigue as they absorb fluctuations into theirsystems over long periods of time. This fatigue adds to both higheroperations and maintenance costs, and shortens the overall unitlifetime.

It is also desirable to have a power source provide voltage support tothe power transmission grid at the point of its interconnection. Suchvoltage support enables the power source to contribute to dampeningvoltage or frequency fluctuations on the transmission line at the pointof power injection. In recent years, power flow controllers have beendeveloped to compensate for transmission fluctuations by injecting apower offset varying in voltage and/or phase angle into the transmissionsystem. An example of one type of power flow controller is described inU.S. Pat. No. 5,808,452 to Gyugyi et al. which employs a dc-to-dcconverter using the dc voltage produced by a first static inverterconnected in shunt with a transmission line to provide parallel reactivecompensation to establish the magnitude of a series compensation voltageinjected into the transmission line by a second static inverter.However, the various techniques for continuous compensation control areusually associated with the following practical disadvantages: increasedcircuit complexity and cost, increased losses, and increased harmoniccontent.

Fluctuations in the power transmission grid can also affect theinterconnection of a power source with the grid. Transient conditionssuch as temporary power outages or flashovers on a transmission line cancause a power sources connected to the grid to become automaticallydisconnected by its safety circuitry, and would thus require a recloseror other relay type device to reconnect the power source back to thegrid once the transient condition has passed. For small-contributorpower sources, such as a wind farm, the addition of a recloser or relaydevice adds an undesirable additional cost to the system. For smallpower systems, such as an island grid, or a weakly supportedinterconnected grid where the wind farm represents a major generationsource (above 5% of total power), if the wind farm is unable toimmediately reconnect to the grid after the fault is cleared (referredto as fault ride through), there may be enough generation/load imbalanceto cause the entire grid to shut down due to underfrequency.

SUMMARY OF INVENTION

It is therefore a principal object of the present invention to provide apower control interface between the power output of an unstable powersource such as a wind farm and a power transmission line which isolatespower fluctuations of the wind farm and prevents the injection ofvoltage or frequency instabilities into the grid during changing windconditions. It is particularly desirable that this be accomplished withrelative simplicity and at low cost while being highly effective incontrolling the effects of both short- and long-term power fluctuationsof the wind farm on the power transmission system. It is another objectof the present invention to have the power control interface provideeffective voltage support to the power transmission line at the point ofinjection of power output from the power source. It is still a furtherobject of the invention that the power control interface provide a faultclearance capability to “ride through” a transient fault condition onthe power transmission line, i.e., allow the power output to remainconnected to the grid during transient fault conditions without the needto add a recloser or relay circuitry to the system.

In accordance with the present invention, a power control interfacebetween a power output of an unstable power source such as a wind farmand a power transmission line comprises:

-   -   (a) an electrical energy storage coupled between the unstable        power source and the power transmission line to store excess        power output when it is above a normal output level of the        unstable power source and to release stored electrical energy to        add to the power output when it is below the normal output level        of the unstable power source;    -   (b) a control system which receives a power source data signal        derived from monitoring the power output of the unstable power        source and a transmission line data signal derived from        monitoring the power transmission line, and which determines        when electrical energy stored in the electrical energy storage        is to be released to add to power output to the power        transmission line to compensate for conditions of decreased        power generation encountered by the unstable power source, or        when excess electrical energy generated during conditions of        increased power generation encountered by the unstable power        source is to be stored in the electrical energy storage; and    -   (c) an electronic compensation module which receives a control        signal from the control system corresponding to its        determination and operates to release electrical energy stored        in the electrical energy storage to add to power output to the        power transmission line to compensate for decreased power source        output, and to store excess electrical energy from increased        power source output in the electrical energy storage in        accordance with said determination.

In a preferred embodiment of the invention, the AC power output of thewind farm may be converted by an ac-to-dc inverter to direct current(DC) for storage in a DC capacitor array, ultracapacitors, or battery.The electrical energy storage is controlled by the electroniccompensation module to act like an “electronic shock absorber”, servingas both an energy source when the power output of the wind farm istemporarily decreasing below its normal range, and as an energy sinkwhen power output from the wind farm is temporarily increasing above itsnormal range. This “shock absorbing” function has the effect ofsmoothing overall fluctuations in the power output of the wind farm andpreventing the injection of frequency and voltage instabilities into thepower transmission system. The electronic compensation module respondsto control signals from the control system by issuing feedback signalsto a pair of complementary “gates” on the input and output ends of theenergy storage in order to store electrical power when the power outputof the wind farm is temporarily decreasing below range, and to releaseelectrical power into the power transmission system when the poweroutput is temporarily increasing above range.

The control system is also provided with a “look ahead” capability forpredicting wind speed conditions and maintaining the energy storage in amode consistent therewith when wind fluctuations are in a “narrow-band”of short duration or small speed changes, in order to avoid out-of-syncresponses due to time lag in the response of the system to real timeconditions. The control system receives data signals derived from thewind farm power output representing information on the voltage, current,power and frequency output of the wind farm, and similar informationderived from monitoring the transmission line. The control systememploys system-modeling algorithms based on historical data of the windfarm output, meteorological data taken from the site, and meteorologicalpredictions of the hour-ahead and day-ahead wind conditions, andcompares current power output information from the wind farm with thecalculations of the algorithms to predict narrow-band wind speedconditions. Based on this information, the control system sends acontrol signal to the electronic compensation module to remain in a modeto release energy from the energy storage to increase power outputinjected into the power transmission line or to store energy in theenergy storage without adding to power output to the transmission lineduring narrow-band wind speed changes.

The power control interface can also provide voltage support for thepower transmission system through the use of power electronics thatconvert DC power to AC and inject it into the transmission line at theproper voltage and phase angle in order to provide either real orreactive power depending on the stability needs of the transmissionsystem. The power control interface can also provide fault clearancecapability by adequately sizing the energy storage and controlling it toenable “riding out” of transient fault conditions occurring on the powertransmission line. This may be particularly useful in island gridsystems of small total capacity where the wind farm output may be asubstantial power source contributor, or where other power sourcecontributors may be weakly interconnected and become lost (disconnected)during a transient outage.

Other objects, features, and advantages of the present invention will beexplained in the following detailed description of the invention havingreference to the appended drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a system diagram illustrating a preferred embodiment of apower control interface between the power output of an unstable powersource such as a wind farm and a power transmission line in accordancewith the present invention.

FIG. 2 is a block diagram illustrating an example of a control systemfor the power control interface.

FIG. 3 is a simplified diagram of an energy storage circuit for thepower control interface.

FIG. 4 is a chart showing the output of a typical wind farmcharacterized by large power fluctuations.

DETAILED DESCRIPTION OF INVENTION

The following describes a preferred embodiment of a power controlinterface between the power output of an unstable power source such as awind farm and a power transmission line to which the unstable powersource is connected. The exemplary embodiment illustrates the functionof the power control interface acting as an “Electronic Shock Absorber”which uses a combination of electrical energy storage and powerelectronics to isolate a power transmission system from wind farm energyfluctuations, while also enabling the wind farm to compensate for powerdisturbances on the line at the point of injection into the powertransmission system. The Electronic Shock Absorber system can thusincrease the overall capacity factor of the wind farm contribution tothe grid, provide voltage support at the point of injection, and improveoverall transmission system stability. The invention offers an uniquesolution to three major problems that have impaired wider use of windenergy as a renewable source of electricity generation.

Referring to FIG. 1, the power control interface of the presentinvention (referred to as the “Electronic Shock Absorber” within thedotted lines in the figure) is coupled between conventional 60 hertz(cycle) AC power output from windmill generators of a wind farm (poweroutput in) and an AC transmission line which transmits power output(out) from the wind farm to a power transmission grid. The three maincomponents of the Electronic Shock Absorber are labeled as “EnergyStorage”, “Control System”, and “Electronic Compensation Module”. TheEnergy Storage is coupled between the power output of the wind farm andthe power transmission line. The Control System receives a Wind FarmData Signal derived from monitoring the wind farm power output, whichprovides information on the voltage, current, power and frequency outputof the wind farm, and a Transmission Line Data Signal derived frommonitoring transmission line conditions, which provides similarinformation on the transmission line. On the basis of the Wind Farm DataSignal and the Transmission Line Data Signal, the Control Systemprovides a control signal to the Electronic Compensation Module toeither store excess energy from rising wind farm power generation abovethe norm or release energy to the power transmission line to compensatefor wind farm power generation below the norm. The ElectronicCompensation Module responds to the control signal from the ControlSystem by issuing feedback signals to a pair of complementary “gates” onthe input and output ends of the Energy Storage, referred to in thefigure as the “Charge Controller” and the “Discharge Controller”. Thesehave the function of either releasing stored energy from the energystorage to compensate for decreasing wind farm power generation oradding electrical energy to the energy storage during rising powergeneration. The Control System employs system-modeling algorithms in a“look-ahead” function (described further below) to predict fluctuationsin the expected power output of the wind farm to control the storage orrelease of power in or from the energy storage when power fluctuationsare in a “narrow-band” of short duration changes.

The capacity of the Energy Storage is sized to balance costconsiderations with having sufficient capacity to accommodate powerfluctuations typically encountered in a given wind farm as well asadditional capacity for “ride through” of transient fault conditions andfor reserve. Since storage capacity is relatively expensive, thecapacity is sized to be about 1.5 times the typical expected maximumpower excursion for the limiting condition for the stability of thepower system, to which the wind farm is connected. Detailed calculationswill have to be performed for each specific application of theElectronic Shock Absorber with considerations given to grid stability,acceptable ramp rate (up and down), necessity for fault ride through,sub-minute power fluctuations tolerance, and detailed wind regimecharacteristics to accurately size the storage component of the system.

The Energy Storage, which is controlled by the Electronic CompensationModule, offsets voltage and frequency changes in the wind farm outputand reduces fluctuations in the power output to the transmission systemin the following manner. When there is a sudden drop in the power outputof the wind farm, for example, the Energy Storage will be operated bythe Electronic Compensation Module to discharge energy to make up forany shortfall in energy output of the wind farm. Conversely, when thepower output of the wind farm suddenly increases, the Control Systemwill cause the Energy Storage to charge, and thereby blocking anyaddition from Energy Storage to the wind farm power output to thetransmission system. This compensation has the effect of smoothing theoverall fluctuation in frequency and voltage on the transmission system.It should be noted that the purpose of the power control interface is tosmooth the power output of the wind farm to the transmission system andnot to provide energy storage for use in off-peak hours, as is normalwith many wind power systems. Because of the unique nature, purpose andcontrol of this energy storage system, its capacity can be limited toenergy storage equivalent to a few tens of seconds of wind farm energyoutput. This relatively small storage capacity significantly reduces theoverall size and capital cost of the Electronic Shock Absorber.

While the Electronic Shock Absorber is operated automatically to smoothpower output fluctuations when the fluctuations are large increases ordecreases (e.g., more than 10%) from normal design-rated wind speedrange and extend over a design-rated time window (e.g., 6 to 10 second),it becomes less efficient to operate the Electronic Shock Absorberautomatically when the wind fluctuations are sharp (e.g., less than 5seconds duration) or small changes (e.g., less than 10% of the normalpower output range). This is because the response of the Control Systemwill have a certain time lag (e.g., 0.1 to 0.2 seconds) in relation toreal time conditions, and operation with automatic energy release/storechanges that lag actual wind speed conditions may result in theElectronic Compensation Module causing the Energy Storage to storeenergy when actual wind speed is falling and release energy when actualwind speed is rising. This type of out-of-sync condition would reducethe overall efficiency of the system and/or might introduce powerfluctuations into the transmission system. For smoothing thesenarrow-band fluctuations, the power control interface is provided with a“look ahead” capability to predict the likely wind speed conditionswithin the narrow band range and maintain the Energy Storage in a modeconsistent with the predicted wind speed conditions despite sharpduration or small wind speed changes.

As illustrated in greater detail in FIG. 2, a preferred embodiment ofthe Control System has a Transmission Line Control Signal Calculatorwhich processes signals derived from monitoring the status of the ACtransmission line, including a fault monitor, voltage magnitude, currentmagnitude, and VARs variations (volt-amperes reactive), and provides aTransmission Line Data Signal to the main Control Calculator. The WindFarm Data Signal is a composite of a Sub-minute Load Limit Calculatorand a Ramp Rate Limit Calculator monitoring the wind farm output, and aWind Prediction Control Signal which is based on Wind Speed Monitor Dataand Historical Wind Profile Data. As used herein, the term “calculator”refers to any means (e.g., a microprocessor) for processing inputsignals in accordance with programmed rules, steps and/or algorithms inorder to determine and issue a corresponding output signal. Based onthese inputs, the Control Calculator determines the appropriate controlsignal for the Control System Output. The Wind Prediction Control Signalis determined in accordance with prediction algorithms (examplesdescribed below) for judging whether current power generation conditionswill rise above or fall below the normal power output range for the windfarm, in order to dampen the system's response to fluctuations in thewind farm power output of short duration. The Control System Output isprovided to the Electronic Compensation Module to maintain the EnergyStorage in a mode consistent with the predicted power output within therange of narrow-band fluctuations.

An example of relevant inputs for the “look ahead” function of the PowerOutput Analysis Module during narrow-band fluctuations is provided inTable I below (actual data would be site specific): TABLE I Current WindFarm Data: Date: Month:_ Day:_ Year:_(—) Current wind speed: _mphCurrent temperature: _degrees F. Current barometric pressure: _mbarWeather day characterization: Clear/Cloudy/HighW/LowW/GustyW Number ofturbines online: _turbines Total current power output: _MW SystemComparison Data: Turbine design rating, normal range: _KVA Wind speeddesign rating, normal range: _mph Number of turbines rated online:_turbines Total design rated power output, normal range: _MW Historicalwind speed/hour: _mph Historical wind speed/day-ave: _mph (ave)Historical temperature: _degrees F. Historical barometric pressure:_mbar Historical weather day characterization:Clear/Cloudy/HighW/LowW/GustyW Power Output Look-Ahead Prediction:Expected wind speed deviation from norm/hour: _mph above/below normExpected wind speed deviation from norm/day-ave: _mph above/below normExpected power deviation from norm/hour: _MW above/below norm Expectedpower deviation from norm/day-ave: _MW above/below norm

The Control System receives the above types of information on currentwind farm conditions and AC transmission line conditions to provide the“look-ahead” function for predicting power output within the range ofnarrow-band fluctuations. As one example, the Control System can employcertain system modeling algorithms to act as a state estimator todetermine whether to store excess generated energy or release storedenergy to the transmission system during narrow-band wind speed changes.This state estimation is used to provide the appropriate control signalsto the Electronic Compensation Module. An example of system modelingalgorithms that might be used for predictive determination for the “lookahead” function is given in Table II below. TABLE II 1. IF: Current dayave. wind speed > 110% normal (design rated) wind speed range OR Currentday ave. wind speed < 90% normal wind speed range THEN: Allow EnergyStorage to operate release/store automatically. 2. IF: 90% < current dayave. wind speed < 110% normal wind speed range AND Current day type =historical day type, AND historical wind speed > normal range THEN: PutEnergy Storage gates in storage mode 3. IF: 90% < current day ave. windspeed < 110% normal wind speed range AND Current day type = historicalday type, AND historical wind speed < normal range THEN: Put EnergyStorage gates in release mode 4*. IF: Fault condition is detected intransmission line data  (* For fault clearance) THEN: Put Energy Storagegates in release mode

In FIG. 3, a simplified example of an Energy Storage circuit used as a“shock-absorber” in the power control interface is shown. The EnergyStorage circuit is controlled by signals from the Charge and DischargeControllers which act like double-action gates controlled by signalsfrom the Electronic Compensation Module in response to the controlsignal received from the Control System. The Energy Storage circuitshown includes an array of ultracapacitors which can store AC power inand release AC power out. A number of stages (here 3 stages) ofcomplementary thyristors acting as inverters are used to store power inthe ultracapacitors in stages. The thyristors are controlled by the gatedrivers GD. When the Energy Storage circuit receives a “charge” signalfrom the Charge Controller, input power is passed through the input LCcircuit to ramp up energy for storage. One stage of complementarythyristors is used to release power from the ultracapacitors. When a“discharge” signal is received from the Discharge Controller, energy isreleased by the output thyristors through the output transformercircuit. The release of power from the Energy Storage is continued aslong as the control signal from the Control System specifies the releasemode.

The number and capacities of the ultracapacitors included in the circuitis determined based on a tradeoff of the shock-absorbing capacitydesired versus storage costs. When the charging level of the EnergyStorage reaches its total capacity, the Charge Controller transitionsthe excess power back to the Main Power Output Node (the Energy Storageis filled). The Energy Storage circuit components are selected so thatsmooth transitions are made in power storage and release without causinganomalies in the system. As an alternative Energy Storage circuit, theAC wind farm output can be converted by an ac-to-dc inverter to directcurrent (DC), and the energy storage is provided by an array of DCcapacitors or battery storage units.

The shock-absorber function of the power control interface of thepresent invention allows the wind farm power output to be stored duringperiods of excess power generation and released to supplement the powerinjected into the AC transmission line during declining powergeneration. Since wind farm output (wind speed and gusting conditions)can vary considerably over short durations, the power control interfaceutilizes the power output predictive function to maintain the energystorage or the energy release mode despite short-duration fluctuations.An example of the high variability of power output of a typical windfarm is shown in FIG. 4 (for the time scale in seconds, 3600 secondsequals one hour).

The Electronic Shock Absorber can also enable a wind farm to helpreinforce the transmission system, i.e., provide voltage support,against power problems originating elsewhere on the transmission system.On a relatively weak transmission system that is thermally limited,i.e., has limited stable, fuel-fired power source contributors,maintaining proper voltage and phase angle in the transmission systemcan be frequently difficult. Ensuring that power flows in the rightdirection can depend on the timely injection of reactive power (measuredin volt-amperes reactive—VARs) at critical points to support the voltageat the injection point. The Electronic Shock Absorber can use energystored in the Energy Storage to make adjustments to the interconnectpower output of the wind farm to supply more VARs as needed to supporttransmission system voltage in the area. Similarly, when powerdisturbances are created elsewhere on an interconnected transmissionsystem, the Electronic Shock Absorber can react quickly to supply shortbursts of real power (measured in Watts) that can help dampen thedisturbances.

To provide voltage support, the power control interface can include anoptional power electronics module that converts stored power intocurrent or phase angle offsets for injection into the transmission lineat the point of interconnect. The enables the power control interface toprovide either real or reactive power, depending on the stability needsof the transmission system. This capability can be provided by powerelectronics circuitry similar to that known in the power industry as a“distribution static compensator” (D-STATCOM), which is a multi-poleinverter based on insulated gate bipolar transistor (IGBT) technologythat injects current at the proper phase angle into a power line tocompensate for power disturbances. The D-STATCOM technology is describedin further detail in “Custom Power: Optimizing Distribution Services”,by John Douglas, in the EPRI Journal, published by the Electric PowerResearch Institute (EPRI), Vol. 21, No. 3, Pages 6-15, May/June 1996,and is incorporated herein by reference. The voltage support capabilitycan be readily implemented by having the Control System issue datasignal inputs to the D-STATCOM power electronics circuitry indicatingthe current voltage level and phase angle on the transmission linederived from the Transmission Line Data Signal, thereby enabling theD-STATCOM power electronics circuitry to generate the necessary offsetsfor injection to the transmission line to compensate for the powerdisturbances.

Energy stored in the Electronic Shock Absorber can also provideride-through capability for the wind farm to allow for fault clearanceon the transmission system. Typically, when a ground fault occurs on atransmission system, a protective relay is used to open the connectionof a power source contributor from the transmission line to protect itsmechanical system from surge for a pre-set interval (generally 6-36cycles) and then re-closes. Most ground faults—such as a tree limbtouching a power line—are cleared by flashover within that interval.During that period, however, a wind farm equipped with a relay andrecloser would be disconnected from the transmission system by its ownprotection devices, thereby potentially worsening the power loss problemin a small (island) transmission system and causing loss of power tocustomers. The Electronic Shock Absorber can be configured to use itsenergy storage capacity to “ride through” the flashover interval. Thiswould obviate the need for the wind farm interconnection point to beequipped with a relay and recloser, and would allow it instead tocompensate for the momentary loss of power on the transmission system.The fault clearance capability can be implemented by having the ControlSystem issue a control signal for Release mode when a transient faultcondition is detected from the Transmission Line Data Signal (see Item4* in Table II).

It is to be understood that many modifications and variations may bedevised given the above description of the principles of the invention.It is intended that all such modifications and variations be consideredas within the spirit and scope of this invention, as defined in thefollowing claims.

1-20. Cancelled
 21. A power control device adapted to be coupled betweena power output of an unstable power source such as a wind farm and apower transmission line, comprising: (a) an electrical energy storagecoupled between the unstable power source and the power transmissionline to store excess power output from the unstable power source asstored electrical energy when it is above an expected output level, andto release stored electrical energy to add power output to the powertransmission line when the power output of the unstable power source isbelow its expected output level; (b) an electronic controller modulecoupled to the electrical energy storage which receives a power sourcedata signal derived from monitoring the power output of the unstablepower source and generates controller signals for controlling theelectrical energy storage indicating when excess power output from theunstable power source above its expected output level is to be stored aselectrical energy in the electrical energy storage, and when electricalenergy stored in the electrical energy storage is to be released to addpower to the power output to the power transmission line.
 22. A powercontrol device according to claim 21, wherein the unstable power sourceis a wind farm providing an AC power output, and the power output isstored in a selected one of the group of electrical energy storagedevices consisting of ultracapacitors, capacitors, and batteries.
 23. Apower control device according to claim 21, wherein the unstable powersource is a wind farm providing an AC power output, and the AC poweroutput is converted by an ac-to-dc inverter to direct current (DC) forstorage in a DC capacitor array or battery.
 24. A power control deviceaccording to claim 21, wherein the electronic controller module includesan unstable power source monitoring means, a transmission linemonitoring means, and a control calculating means for calculating whatthe power output of the unstable power source is likely to be over anarrow-band range and for maintaining the release of energy or thestorage of energy in the electrical energy storage over the narrow-bandrange despite power fluctuations of short duration.
 25. A power controldevice according to claim 24, wherein the unstable power source is awind farm, and the controller module receives data signals derived frommonitoring the power output of the wind farm, and data signals derivedfrom monitoring conditions on the transmission line, and the controlsystem employs system-modeling algorithms to predict narrow-band windspeed conditions.
 26. A power control device according to claim 25,wherein the controller module's algorithms include prediction based oncurrent data on wind farm output and historical data of wind farmoutput.
 27. A power control device according to claim 21, wherein theelectrical energy storage includes an energy storage circuit, a chargecontroller at an input side of the energy storage circuit for receivingelectrical energy therein, and a discharge controller at an output sideof the energy storage circuit for electrical energy release.
 28. A powercontrol device according to claim 27, wherein the charge controller andthe discharge controller act like double-action gates controlled by thecontroller signals generated by the controller module.
 29. A powercontrol device according to claim 27, wherein the unstable power sourceis a wind farm providing an AC power output, and the power output isstored in a selected one of the group of electrical energy storagedevices consisting of ultracapacitors, capacitors, and batteries.
 30. Apower control device according to claim 29, wherein the electricalenergy storage is an array of ultracapacitors arranged to storeelectrical energy in respective stages.
 31. A power control deviceaccording to claim 21, further comprising power electronics circuitryfor providing voltage support at the point of connection of the windfarm with the power transmission line.
 32. A power control deviceaccording to claim 31, wherein the power electronics circuitry forproviding voltage support is distribution static compensator (D-STATCOM)circuitry.
 33. A power control device according to claim 21, wherein thecontroller module includes a control mode for fault clearance capabilityfor “riding through” a fault condition on the power transmission lineusing electrical energy stored in said electrical energy storage.
 34. Apower control device according to claim 33, wherein the controllerdevice receives data signals derived from monitoring the powertransmission line, and enables the electrical energy storage to releasestored electrical energy to maintain power output to the powertransmission line when a fault condition on the power transmission lineis detected.
 35. A method of operating a power control device adapted tobe coupled between a power output of an unstable power source such as awind farm and a power transmission line, comprising the steps of: (a)monitoring the power output of the unstable power source; (b)determining when the power output of the unstable power source is aboveits expected output level, and generating a first controller signalindicating that excess power output from the unstable power source aboveits expected power output level is to be stored as electrical energy inan electrical energy storage; and (c) determining when the power outputof the unstable power source is below its expected output level, andgenerating a second controller signal indicating that stored electricalenergy stored in the electrical energy storage is to be released to addpower to the power transmission line.
 36. A method of operating a powercontrol device according to claim 35, further comprising the step ofcalculating the likely power output of the unstable power source over anarrow-band range, and generating an appropriate controller signal formaintaining the release of electrical energy from the electrical energystorage or the storage of electrical energy in the electrical energystorage over the narrow-band range despite power fluctuations of shortduration.
 37. A method of operating a power control device according toclaim 36, wherein the power source is a wind farm, and the step ofcalculating the likely power output of the wind farm and generating anappropriate controller signal is based on monitoring current poweroutput of the wind farm and using historical wind farm data, andemploying system-modeling algorithms to predict narrow-band wind farmoutput based thereon.
 38. A method of operating a power control deviceaccording to claim 36, wherein the power source is a wind farm, and thestep of calculating the likely power output of the wind farm andgenerating an appropriate controller signal is performed so as toautomatically smooth power output fluctuations when the fluctuations arelarge increases or decreases from a design-rated wind speed range andextend over a design-rated time window in order to prevent operation inan “out-of-sync” condition.
 39. A method of operating a power controldevice according to claim 36, wherein the power source is a wind farm,and the step of calculating the likely power output of the wind farm andgenerating an appropriate controller signal is performed so as toprovided a “look ahead” capability to predict the likely wind speedconditions within the narrow band range and maintain the power outputreleased from or stored in the electrical energy storage in a modeconsistent with the predicted wind speed conditions despite sharpduration or small wind speed changes.
 40. A method of operating a powercontrol device according to claim 36, wherein the power source is a windfarm, and the step of calculating the likely power output of the windfarm and generating an appropriate controller signal is performed with aTransmission Line Control Signal Calculator which processes signalsderived from monitoring the status of the power transmission line, and aWind Prediction Control Signal is generated based on Wind Speed MonitorData and Historical Wind Profile Data for judging whether current powergeneration conditions will rise above or fall below the normal poweroutput range for the wind farm.